1. Field of the Invention
The present invention relates to a data demodulator and more particularly to a data demodulator feasible for a receiver included in a digital radio communication apparatus.
2. Description of the Background Art
Generally, a digital radio communication apparatus using, e.g., a PSK (Pulse Shift Keying) modulation system includes a receiver for receiving a radio wave with an antenna. A radio wave coming in through the antenna is input to a first or front end band-pass filter for removing spurious signal components. The band-pass filter has its output connected to a mixer, which is adapted to multiply the output of the band-pass filter by a preselected frequency signal. The mixer has its output connected to a decision circuit, which is adapted to convert the resulting output of the mixer to an IF (Intermediate Frequency) signal that can be demodulated. The IF signal is fed to a second band-pass filter so as to further reduce noise components, i.e., to increase the ratio of a necessary modulated component to the entire signal. The second band-pass filter has its output connected to a decision circuit, which is adapted to demodulate the output of the second band-pass filter to thereby output a demodulated digital signal. The decision circuit may be adapted to use synchronous detection, delay detection or similar conventional technology detection scheme.
FIG. 1A shows the frequency spectrum of a specific modulated wave received via the antenna. FIGS. 1B, 1C and 1D respectively show the outputs of the first band-pass filter, the mixer, and the second band-pass filter in frequency spectrum.
The receiving ability of the above-described receiver depends not only on the detection system and circuit arrangement of the decision circuit, but also on the characteristic of the second band-pass filter expected to attenuate noise outside of a frequency band assigned thereto. It is generally ideal for the second band-pass filter, if provided with, e.g., a Nyquist filter characteristic, to be adapted to pass a modulated component only with the remaining frequency components filtered out, along with its fidelity.
The problem with the above-described conventional receiver will be described specifically with reference to FIGS. 2A and 2B. FIGS. 2A and 2B respectively show an ordinary band-pass filter characteristic and an ideal band-pass filter characteristic available with the conventional system. The receiver would be able to cancel all noise components ascribable to spurious frequency components if provided with an ideal band-pass filter. However, as shown in FIG. 2B, even that receiver cannot cancel a noise component superposed on the pass band of the filter. A bit error rate is determined by a power ratio between the signal component and the noise component that cannot be cancelled. While the bit error rate depends on the modulation system, the conventional demodulation system cannot reduce the noise component contained in the frequency band of the demodulated wave, and is therefore unable to realize a receiver characteristic superior to the theoretical value of any one of the modulation systems.